1. Field of the Invention
The present invention relates to ionic compounds of use in the preparation of an electrolyte for batteries.
2. Description of Related Art
Electrolyte solutions in a nonaqueous medium, in particular a nonprotogenic medium, more commonly referred to as an “aprotic” medium, are of great technological importance as they make it possible to extend the potential range in which a battery can operate without side reactions, such as the decomposition of the solvent, said potential not exceeding the value of 1.3 V in water.
The media capable of dissolving salts are mainly polar organic solvents or solvating polymers, in particular those comprising ether groups distributed in a macromolecular chain, the architecture of which can be linear or branched, of comb type, having or not having crosslinking nodes. The polyethers having —CH2CH2O— repeat units are particularly valued for their high solvating power.
Ionic liquids are also known, which products are salts which are molten at low temperature and which are composed of at least one cation possessing a delocalized charge, such as (ethyl)(methyl)imidazolium (EMI), (methyl)(propyl)-pyrrolidinium or diethylmethyl(2-methoxyethyl)ammonium, and of an anion, preferably itself also possessing a charge delocalized over a large volume, in order to reduce the interactions between the cations and the anions and to thus make it possible to achieve low solidifying temperatures.
The solutes intended to introduce the conductivity of ionic type required for the electrolytes are chosen from metal salts and from “ium” salts obtained by binding of a free electron pair of one or more elements, such as N, O, S, P, As or I, with a proton or an organic radical, to form a cation. Mention may be made of ammonium, phosphonium, sulfonium, iodonium, pyridinium, oxazolium and thiazolium ions. Particular importance is given, among metals, to alkali metal and alkaline earth metal salts, in particular to lithium salts. The lithium ion in fact has a very rich electrochemistry, making it possible to form batteries having a high energy density which are very important in current technology. Mention may be made, as other applications of nonaqueous electrolytes, of electrochromic systems and supercapacitors.
The anions which act as countercharge to the cations are chosen from those which exhibit a delocalized negative charge as aprotic electrolytes cannot form hydrogen atoms with negative charges and delocalization is the only means of obtaining appreciable dissociation under these conditions. Mention may be made, among the most well-known anions, of ClO4−, BF4−, PF6−, AsF6− or SbF6−. The ClO4− anion can form explosive mixtures. The anions derived from As and Sb are toxic and uncommon. The BF4− anion is relatively slightly dissociated. The salts of the LiPF6 anion are the most widely used salts in lithium generators, despite major disadvantages: i) they are very readily hydrolyzable, releasing HF, which is toxic and corrosive with regard to the electrode materials. HF releases cations (Mn, Fe, and the like) from the positive electrode and allows them to migrate to the negative electrode, where they are reduced (Mn°, Fe°, and the like), which significantly increases the interfacial impedance of this electrode, reducing the power available and the lifetime; ii) the acid/base equilibrium LiPF6LiF+PF5 releases a very powerful Lewis acid capable of inducing carbocationic chemistry destructive in particular to esters or ethers from which the electrolytic solvent may be formed; iii) in the event of an uncontrolled reaction (“runaway reaction”) with strong heating, LiPF6 can act as fluorinating agent, giving monofluoroethanol or monofluoroacetic acid derivatives, which are excessively toxic.
A fluorine-free coordination anion is also known, in particular bis(oxalato)borate [B(C2O4)2]−, which employs inexpensive elements; however, its lithium salt has a limited conductivity. The rigidity of the anion and its large size give it an unfavorable phase diagram in standard electrolytes comprising ethylene carbonate (poor conductivity at low temperature). Furthermore, this anion has a very limited stability toward oxidation at high temperature (65° C.), which causes problems of self-discharge and of release of gas.
Other anions are known which exhibit high electrochemical stability and high conductivities, both in liquids and in polymers. Among these, the anions capable of forming an anionic liquid are the most effective. The main family is that of the sulfonimides [(RFSO2)2N]−, the most important representative of which corresponds to RF=CF3 (TFSI). The disadvantages of these salts are due, on the one hand, to the absence of passivation of the aluminum above 3.6 V vs. Li+:Li° when the salts are used in batteries or supercapacitors which have an electrode having a current collector made of aluminum. Another disadvantage is the high preparation cost, related to the price of the CF3SO2 synthon. The [(FSO2)2N]− anion would have a more favorable behavior with regard to the corrosion of the aluminum but it is very expensive to prepare and the stability of the lithium salt is limited (130° C.). Generally, it appears that the corrosion of the aluminum is inevitable above 3.6 volts when the electrolyte comprises a salt of a covalent anion as a soluble aluminum salt (such as, for example, the TFSI salt [(CF3SO2)2N]3Al, which is stable and very soluble) can be formed which does not make it possible to passivate the surface of the metal. On the other hand, a coordination anion, such as PF6−, does not form (PF6)3Al but the AlF3 salt, which is insoluble and passivating.
Other anions, “Hückel anions”, are based on the adaptation of the Hückel (4n+2) rule, which predicts the stability of aromatic systems, applied to rings comprising five atoms, the negative charge of which is highly favored. The best known of these anions is 4,5-dicyanotriazole (DCTA):
This purely covalent anion can be regarded as having a 6 “π” electron configuration or a 10 “π” electron configuration, according to whether or not the electrons of the C≡N bonds of the nitrile groups are taken into account, each of these configurations being stable. DCTA salts are thermally stable up to 300° C. Furthermore, the DCTA anion does not comprise fluorine and it is easily manufactured from an industrial precursor, diaminomaleonitrile (DAMN):
However, this anion has the disadvantage of a relatively modest conductivity of its lithium salt (2.9 mS·cm−1 in EC-DMC 50/50) and in particular an oxidation potential of 3.7 V vs. Li+:Li°, which limits its use in a totally unacceptable way for electrode materials such as transition metal oxides LixTM2 (0≦x≦1) with TM=Mn, Ni or Co, the manganese phosphate LiMnPO4 or its solid solutions with the iron phosphate LiMn1-yFeyPO4 (0≦y<1). Even for the iron phosphate (y=1) having a potential of 3.5 V vs. Li+:Li°, the safety margin at the end of charging the electrode is too small.
Salts of anions corresponding to the formula:
in which R is an electron-withdrawing group, for example a perfluroalkylsulfonyl group or a perfluoroalkylcarbonyl group, are known, in particular from EP-0 850 933-A. However, despite the high attractive power of the R group, the presence of oxygen (C═O and O═S═O), which gives very strong interactions with the cations, limits the dissociation. In addition, the C═O or S═O groups are conjugated with the ring and the number of “it” electrons is a multiple of 4. The result of this is that the systems are “antiaromatic” and that they thus have a lower stability toward oxidation and toward reduction. Furthermore, the preparation of this type of compound is very difficult and cannot be carried out in a single stage starting from DAMN.
The synthesis of 2-trifluoromethyl-4,5-dicyanoimidazole is described by M. Bukowska et al, [Polish J. Chem., 78, 417-422 (2004)]. The corresponding lithium salt can be obtained by reaction with lithium carbonate.